Herpesviruses are DNA-containing enveloped viruses belonging to the Herpesviridae family. Among these viruses, HSV-1 and HSV-2 are responsible for recurrent orolabial and genital infections, VZV is responsible for chickenpox and shingles and CMV is associated with disseminated infections such as pneumonitis, colitis and retinitis in immunocompromised individuals. All herpesviruses establish lifelong latent infections in humans with periodic symptomatic and asymptomatic reactivations.
Antiviral agents are commonly used to treat frequent HSV reactivations (mainly genital infections) in immunocompetent individuals, VZV reactivation (shingles or zoster) in elderly subjects and severe HSV, VZV and CMV infections in immunocompromised hosts such as HIV-infected patients, transplant recipients and subjects with neoplasia. Active compounds currently available for the treatment of herpesvirus infections include the nucleoside analogues acyclovir (ACV and its prodrug valacyclovir or VACV), penciclovir (PCV and its prodrug famciclovir), and ganciclovir (GCV and its prodrug valganciclovir or VGCV) as well as the nucleotide analogue cidofovir (CDV) and the pyrophosphate analogue foscarnet (FOS). All these compounds interfere with viral DNA synthesis by inhibiting the viral DNA polymerase (pol). The nucleoside and nucleotide analogues must be phosphorylated to exert their antiviral effect whereas the pyrophosphate analogue FOS directly inhibits viral DNA pol. The nucleoside analogues ACV and GCV are considered the first-line drugs against HSV/VZV and CMV diseases, respectively, whereas CDV and FOS are considered second-line drugs due to their important toxicity (nephrotoxicity and electrolyte imbalances) and lack of oral formulations.
The emergence of herpesvirus drug resistance is frequent in immunocompromised subjects and only surpassed by that of HIV resistance [1]. For instance, HSV resistance to ACV, the first-line drug, varies from 4 to 14% among various immunocompromised groups, although it is infrequent in immunocompetent subjects [1]. The rate of CMV resistance to the gold standard GCV is also substantial with 7% resistance after 6 months of treatment in HIV-infected patients [2] and up to 30% in lung transplant recipients [3]. Due to less frequent use, fewer data on CMV resistance to CDV and FOS have been reported. Some CMV studies reported resistance rate to FOS and CDV similar or higher to that observed with GCV [4,5]. Resistance to the nucleoside analogues can be conferred by alterations in the viral activating gene (HSV/VZV thymidine kinase or CMV UL97 protein kinase) or in the target gene, i.e. the viral DNA pol. Resistance to CDV and FOS arises from mutations in the viral DNA pol gene only. Notably, some mutations in the viral DNA pol gene can confer resistance to all currently-available antiviral agents [6,7]. Thus, there is an urgent need to develop other non-toxic and highly effective compounds with different mechanisms of action to inhibit herpesvirus replication.
The aim of this project was to identify compounds with high binding potential to the DNA pol of HSV-1 by using a strategy that combines 3D modeling and virtual screening of a large bank of compounds. The antiviral activity and toxicity of the top leading compounds were examined in vitro against HSV-1, HSV-2, VZV and CMV. Recombinant and clinical isolates suceptible and/or resistant to current antivirals prescribed for the treatment of herpesvirus infections were used to determine the antiviral potential of these new compounds.